The invention relates to a process for controlling a gas phase oxidation reactor for preparation of phthalic anhydride.
Phthalic anhydride (PA) is prepared industrially by the catalytic gas phase oxidation of aromatic hydrocarbons, such as o-xylene and/or naphthalene, in fixed bed reactors. In general, a mixture of an oxygenous gas and the starting material to be oxidized is passed through tubes in which there is a bed of a catalyst. For temperature control, the tubes are surrounded by a heat carrier medium, for example a salt melt.
Even though the excess heat of reaction is removed by the heat carrier medium, local temperature maxima (hotspots) may develop in the catalyst bed, in which there is a higher temperature than in the remaining part of the catalyst bed. These hotspots lead to side reactions, such as the total combustion of the starting material, or to the formation of undesired by-products which are removable from the reaction product only with great difficulty, if at all.
The hotspot temperatures in the oxidation of o-xylene to phthalic anhydride are typically in the range between 400 and 500° C. Hotspot temperatures above 500° C. are an expression of increased total oxidation of the o-xylene to CO, CO2 and water, and lead to increased damage to the catalyst. Excessively low hotspot temperatures are associated with insufficient conversion of o-xylene, and too high a content of disruptive underoxidation products, for example phthalide, which impairs the product quality. The hotspot temperature depends on a series of parameters, including the o-xylene loading of the gas stream, the loading of the catalyst with the gas stream, the service life of the catalyst, the heat transfer conditions in the reactor and in the salt bath, and the salt bath temperature.
The salt bath temperature is an important correcting parameter for the operation of the gas phase oxidation reactor. It is set correctly when overoxidation or total oxidation proceeds only to a small degree, and the product quality is impaired by underoxidation products to a minimum degree. Excessively high salt bath temperatures lead to falling PA yields and accelerated catalyst aging; excessively low salt bath temperatures result in poor product quality.
In modern plants, the salt bath temperature is controlled by a computer-based process control system, the aim of which is exact compliance with desired operating states. With the aid of mathematical models, the influence of the change of control parameters, such as the phthalic anhydride yield, the conversion of the aromatic hydrocarbon, a hotspot temperature and/or a content of at least one underoxidation product in the reaction product is assessed. Measured values for one or more control parameters are used to determine correcting interventions for control of the control parameter. In addition to the salt bath temperature, important correcting parameters include the loading of the gas stream with the hydrocarbon to be oxidized and the volume flow rate of the gas stream.
The activity of the catalysts or catalyst systems used for gas phase oxidation decreases with increasing operating time. One effect of the thermal stress in the region of the hotspot is the deactivation of the catalyst at the same point. A higher proportion of unconverted hydrocarbons or of partly oxidized intermediates gets into regions of the catalyst bed further downstream. The reaction shifts increasingly toward the reactor outlet and the hotspot migrates downstream. Since downstream catalyst layers are generally more active but less selective, undesired overoxidation and other side reactions increase. Overall, the product yield or selectivity falls with the operating time.
The catalyst deactivation can be counteracted to a limited degree by increasing the temperature of the heat carrier medium, typically at essentially constant hourly space velocity over the catalyst.
The service life of PA catalysts is typically about 5 years, the PA yield within this period decreasing by up to 6% by mass; cf. M. Galantowicz et al. in B. Delmon, G. F. Froment (eds.), Catalyst Deactivation 1994, Studies in Surface Science and Catalysis 88, Elsevier, p. 591-596. Typically, the salt bath temperature is raised by up to 40 K over the lifetime of the catalyst; cf. G. C. Bond, J. Chem. Tech. Biotechnol. 68 (1997) 6-13. For control of the salt bath temperature over a period of several years of catalyst lifetime, the prior art gives only a little information, especially for high catalyst hourly space velocities.
According to the disclosure of DE 2948163 (Nippon Shokubai), the PA yield is 113.8% by mass over a two-layer catalyst at an o-xylene loading of 83 g/m3 (STP) after 2 months at a salt bath temperature of 370° C. After 12 months, at the same o-xylene loading but a salt bath temperature of 375° C., the PA yield decreases to 112.7% by mass. The salt bath temperature in this case was raised by 5 K within 10 months, i.e. by an average of 0.5 K/month.
Anastasov, Chemical Engineering and Processing 42 (2003) 449-460, studied the deactivation of a 04-26 V2O5/TiO2 catalyst from BASF. At an o-xylene loading of 50 g/m3 (STP), the PA yield of 79.9 mol % after 8.5 months at a salt bath temperature of 360° C. rose to 80.7 mol % after 24 months at the same o-xylene loading but a salt bath temperature of 370° C. The salt bath temperature was raised in this case by 10 K within 15.5 months, i.e. by an average of 0.65 K/month. The significant rise in the salt bath temperature by several kelvin per year is thus customary in the oxidation of o-xylene to PA.
A controlled process for temperature control of salt bath reactors for phthalic anhydride synthesis was described for the first time in DE 4109387 (Buna AG). The process comprises the determination of an optimal salt bath temperature from experimentally determined parameters, such as the hotspot temperature and the o-xylene concentration at the reactor inlet. The catalyst aging behavior is taken into account with a linear approach, which employs the apparent activation energy of the catalyst. The salt bath temperature is then adjusted in each case according to the optimal salt bath temperature determined. Depending on the operating conditions, this gives rise to increases or decreases in the salt bath temperatures. At an o-xylene loading of, for example, 43 g/m3 (STP) after 282 days at a salt bath temperature of 376° C., the salt bath temperature was increased to 388° C. after 1470 days at a similar o-xylene loading of 42 g/m3 (STP). This corresponds to an average increase in the salt bath temperature of 0.3 K/month. The optimal salt bath temperature determined after 1470 days of run time was 383° C., and thus 5 K lower. After a lowering of the salt bath temperature by 5 K, the PA yield rose from 65.5 to 71.8 mol %. The o-xylene concentration is included in the formula for determination of the optimal salt bath temperature. The o-xylene loadings reported in the examples are, however, comparatively low at 21-43 g/m3 (STP). In addition, a disadvantage of the method described is that the salt bath temperatures have to be adjusted very frequently.
EP-A 2 009 520 (Honeywell International Inc.) discloses a multivariable process control system for PA preparation. A catalyst performance-dependent first parameter and, as second parameter, the temperature at several positions in the reaction tube are measured. By means of a dynamic model, the temperature is adjusted automatically. The process is said to allow a dynamic adjustment of the temperature profile to compensate for catalyst aging.